In mobile communication networks, orthogonal multiple access, in which signals do not interfere with each other, is widely used for communication between base stations and user devices (e.g., mobile stations). In orthogonal multiple access, different radio resources are allocated to different user devices. Examples of orthogonal multiple access include CDMA (code division multiple access), TDMA (time division multiple access), and OFDMA (orthogonal frequency division multiple access). For example, Long Term Evolution (LTE), which is standardized by the 3GPP, uses OFDMA for downlink communication. In OFDMA, different frequencies are allocated to different user devices.
Non-orthogonal multiple access (NOMA) has been proposed as a scheme for communication between base stations and user devices (e.g., see Patent Document 1). In non-orthogonal multiple access, the same radio resource is allocated to different user devices. More specifically, a single frequency is simultaneously allocated to different user devices. When non-orthogonal multiple access is applied to downlink communication, a base station transmits a signal with high transmission power to a user device with large path loss or low reception SINR (signal-to-interference plus noise power ratio) (that is, in general, a user device located at the edge of a cell area). To a user device with small path loss or high reception SINR (that is, in general, a user device located in the center of the cell area), the base station transmits a signal with low transmission power. Signals to be received by each user device are therefore subject to interference from signals directed to other user devices.
In this case, each user device utilizes a power difference to demodulate a signal directed to the user device. Specifically, each user device first demodulates a signal having the highest reception power. Since the demodulated signal is a signal directed to a user device that is located at the farthest edge of the cell area or that is with the lowest reception SINR, the user device located at the farthest edge of the cell area or with the lowest reception SINR finishes demodulation. Each of the other user devices uses an interference canceller to eliminate, from the received signals, an interfering component corresponding to the demodulated signal. Each of the other user devices then demodulates a signal having the second highest reception power. Since the demodulated signal is a signal directed to a user device that is located at the second farthest edge of the cell area or that is with the second lowest reception SINR, the user device located at the second farthest edge of the cell area or with the second lowest reception SINR finishes demodulation. By repeating demodulation and elimination of high-power signals as described above, every user device can demodulate a signal directed to its own device.
A combination of non-orthogonal multiple access with orthogonal multiple access can increase the capacity of a mobile communication network in comparison with the use of orthogonal multiple access alone. That is, while the use of orthogonal multiple access alone does not allow simultaneous allocation of a radio resource (e.g., a frequency) to multiple user devices, a combination of non-orthogonal multiple access and orthogonal multiple access will allow simultaneous allocation of a radio resource to multiple user devices.
In LTE Advanced, a technique for reception by mobile communication terminals, which is called interference rejection combining, is discussed (e.g., see Patent Document 2). Interference rejection combining (IRC) is a technique for downlink communication in which a user device assigns weights to signals captured by receiving antennae so as to suppress interference, to a desired radio beam from a serving base station (desired base station), from an interfering radio beam from an interfering base station. IRC particularly improves the reception quality of a desired signal carried on a desired radio beam when a user device is located near a boundary of a serving cell area (i.e., a cell area of a desired base station) and is subject to a strong interfering radio beam from another base station (interfering base station) adjacent to the desired base station.